Mastalab — #mastodon client

Silence — P2P #encrypted #SMS

FreeOTP — #OTP code generator

OpenKeyChain — #OpenPGP key manager (integrates with K-9)

Offline Calendar

K-9 Mail — #email client, supports #encryption

Markor — #markdown editor

Sound Recorder

μMath+ — Computer #alegbra system #CAS

Barcodegen — #Barcode generator

SimpleTask Cloudless — #Todo / #TaskManager

(Not from F-Droid): Firefox #mobilebrowser
https://wiki.mozilla.org/Mobile/Platforms/Android